Connecting a load to, or disconnecting it from, a live source of electrical power, such as an electrical voltage bus, may cause undesirable transients on the bus. The transients may interfere with normal operation of other devices connected to the source or they may cause damage to one or more of the devices. For example, FIG. 1 shows an assembly 10a connected to receive energy from an input voltage source 20. The assembly and the input source may be connected by means of connectors 30a, 30b. The assembly comprises a load 14 and a bypass capacitor 12. The bypass capacitor 12 is connected across the load 14, primarily for load regulation; it also helps mitigate the effects of series impedances when source 20 and the load 14 are connected (e.g., source inductance Lb 15 and source resistance Rb 16; assembly inductance La 17 and assembly resistance Ra 18). When the assembly 10a is initially connected to the source 20, the discharged filter capacitor 12 will cause a reduction in the bus voltage Vb, the magnitude and duration of the reduction being a function of the size of the filter capacitor and the relative magnitudes of the series impedances. A transient reduction in Vb may impact operation of other devices (e.g., device 10b) that are connected to the power bus (e.g., by means of connectors 31a, 31b).
One way to manage the effects of connecting a load to a power bus is shown in FIG. 2. In the Figure, an assembly 40a comprises a MOSFET 44 that is connected in series between the input source 20 and the load 14 and bypass capacitor 12. When the assembly 40a is first connected to the bus (e.g., by means of connectors 30a, 30b), a FET Controller 42 in the assembly 40a controls the conductivity of the MOSFET 44 to control the initial charging of the bypass capacitor 12. By controlling the rate of charge of the capacitor 12, a reduction in the bus voltage Vb associated with the charging of the capacitor may be reduced. Once the capacitor is sufficiently charged, the FET Controller 42 may fully turn ON the MOSFET (i.e., bias it into a very low resistance state), thereby effectively connecting the capacitor 12 and the load 14 directly to the bus voltage Vb.
A sudden turning OFF of the switch 44 in the assembly 40a of FIG. 2, as might be effected by the FET Controller 42 in the event of a load 14 fault (such as a short circuit or other overload condition), may cause an uncontrolled transient increase in the voltage Vd at the drain of the MOSFET, and in the bus voltage Vb, owing to the energy stored in series parasitic inductances (e.g. inductances Lb 15, La 19, FIG. 2). The waveforms of FIG. 3, for example, show the effect of a sudden turning OFF of the MOSFET in response to a short circuit in the load 14 for the assembly of FIG. 2 comprising a generic MOSFET modeled with a 100 volt breakdown characteristic and having an ON resistance of RdsON=5 milliohms; an input source voltage of 12 VDC; a total series inductance La+Lb=0.6 microhenry; and a total initial series resistance Ra+Rb=2 milliohms. In FIG. 3 the short circuit occurs at time t=0 and an overcurrent condition is detected (by a current sensor circuit, not shown in FIG. 2) at time t=2.5 microseconds, when the current Ia has increased to approximately 50 Amperes. In response to the overcurrent condition, the FET Controller rapidly turns the MOSFET OFF by sinking a 100 mA gate discharge current, Ig (FIG. 3D), from the gate of the MOSFET (the MOSFET gate-to-source voltage, Vgs, is shown in FIG. 3B). As shown in FIGS. 3A and 3C, the rapid discharging of the gate causes a rapid and essentially uncontrolled increase in the MOSFET drain voltage, Vd (FIG. 3C), so that, by time t1, Vd increases to approximately 100 Volts and the MOSFET enters avalanche breakdown. The rapid and uncontrolled transient increase in the voltage Vd, and bus voltage Vb, may cause failure of voltage-sensitive components, such as integrated circuits, in this assembly or in other assemblies that receive the bus voltage (e.g., assembly 40b of FIG. 2), or it may damage the MOSFET 44.
As shown in FIG. 4, one way to control the peak magnitude of a transient increase in the voltage Vd is to install a voltage limiting circuit or device (e.g., a Transzorb 46 or a Metal-oxide varistor (not shown)) at the input of the assembly 40, and preferably close to the drain of the MOSFET 44. Apparatus and methods for active overvoltage protection are also described in Bruckmann et al, U.S. Pat. No. 6,407,937, “Active Overvoltage Protection Apparatus for a Bidirectional Power Switch in Common Collector Mode”, and in Kelly, U.S. Pat. No. 5,401,996, “Overvoltage Protected Semiconductor Switch.”
As used herein, the term “hot swap” refers to connecting a load to, or disconnecting it from, a live source of electrical power, such as an electrical voltage bus. In hot swap applications, care must be taken to ensure that hot swapping one assembly does not affect other assemblies on the bus, e.g., by causing the voltage on the bus to increase or decrease to an extent that would impact the operation of another assembly or cause damage. Hot swapping may refer to physically connecting or disconnecting an assembly (e.g., plugging an electronic assembly into, or unplugging an electrical assembly from, a connector; using a mechanically actuated switch to connect or disconnect the assembly) or it may refer to electronically connecting or disconnecting the assembly (e.g., by means of a controlled semiconductor device, such as a MOSFET switch).
Switching behavior, operating regions and gate drive characteristics of MOSFET transistors are described in AN-1084, “Power MOSFET Basics”; in AN-937, “Gate Drive Characteristics and Requirements for Power MOSFETs”; and in AN-947, “Understanding HEXFET Switching Performance”; all by International Rectifier Corporation, El Segundo, Calif., USA. MOSFET terminology is illustrated with reference to FIG. 16, which shows an example of current-voltage characteristics for an N-channel enhancement-mode MOSFET. Each of the curves labeled 1, 2, 3 and 4 show current-voltage characteristics at different values of gate-to-source voltage Vgs. The region to the right of the dashed line 300 is conventionally referred to as the “saturation” region of the MOSFET; the region to the left of the dashed line 300 is conventionally referred to as the “ohmic”, “linear” or “triode” region of the MOSFET. In the “linear” region the resistance of the MOSFET approaches a relatively very low value that is relatively insensitive to variations in Vgs; when operating in the saturation region, however, the MOSFET drain current is relatively very sensitive to changes in Vgs. As used herein with reference to a MOSFET, the term “ON” refers to operation of the MOSFET deep in the “linear” region—e.g., in the region labeled 302 in FIG. 16. When ON, the “ON resistance” (RdsON) of a MOSFET is typically characterized by the manufacturer at a specific combination of Vgs and drain current.